1. Field of the Invention
The present invention generally relates to an optical film and a light source module, and more particularly, to a prism sheet and a backlight module using the prism sheet.
2. Description of Related Art
FIG. 1A is a partial cross-sectional diagram of a conventional backlight module and FIG. 1B is a three dimensional diagram of the prism sheet in FIG. 1A. Referring to FIGS. 1A and 1B, a conventional backlight module 100 includes a reflective sheet 110, a plurality of cold cathode fluorescent lamps (CCFLs) 120, a bottom diffuser 130, a prism sheet 140 and a top diffuser 150, all of which are sequentially disposed from the rear side to the front side of the backlight module 100. The CCFLs 120 are capable of emitting a light beam 122. A part of the light beam 122 strikes the reflective sheet 110, and then is reflected by the reflective sheet 110 onto the bottom diffuser 130 and transmitted to the prism sheet 140. Another part of the light beam 122 directly strikes onto the bottom diffuser 130 and is transmitted onto the prism sheet 140.
The prism sheet 140 includes a transparent substrate 142 and a plurality of prism rods 144 disposed on the transparent substrate 142. Each of the prism rods 144 extends along a first direction D1, and the prism rods 144 are arranged along a second direction D2. The prism rods 144 have selectivity on the incident light beams 122 with different incident angles. That is, the prism rods 144 allow an incident light beam 122 with an incident angle within a particular angle range to be passed through, so that the light beam 122 emitted out of the prism sheet 140 is as perpendicular as possible to the top diffuser 150. Therefore a light collecting effect of the prism sheet 140 is achieved, and the backlight module 100 thereby is able to provide a surface light source with more concentrative light emitting angles. For example, the light ray 122a of the light beam 122 may pass through the prism rods 144 and reach the top diffuser 150. However, the light rays 122b and 122c of the light beam 122 would be reflected by the prismatic surfaces 114a and 144b of the prism rods 144 back onto the reflective sheet 110. Then, the reflective sheet 110 reflects the light rays 122b and 122c onto the prism sheet 140 so as to reuse the light rays 122b and 122c. The prism rods 144 allow a part of the above-mentioned reused light beam 122 to be passed through and reflect another part of the reused light beam 122 once more. So a part of the light beam 122 circulates between the prism rods 144 and the reflective sheet 110 many times until coming to an incident angle close to the angle of incidence of the light ray 122a and passing through the prism rods 144.
Since the light beam 122 emitted from the CCFLs 120 is a white light beam containing a plurality of wavelengths, and since the prism rods 144 have different refractive indexes depending on different wavelengths of light, a dispersion phenomenon of the light beam 122 passing through the prism rods 144 occurs, which leads to color nonuniformity of the surface light source provided by the backlight module 100.
In addition, since the profile of the prism rods 144 (i.e., the crest lines LI at the top of the prism rods 144 and the boundary lines L2 between any two adjacent prism rods 114) is apparent, the prism rods 144 and the pixel array (not shown) of the liquid crystal panel disposed over the backlight module 100 generate moire fringes or Newton's rings, which further results in nonuniform brightness of the surface light source provided by the backlight module 100. Besides, if the prism rods 144 with the apparent profile have defects, the defects are more easily noticeable.